The invention relates to the field of wind turbine generators. Specifically, the invention relates to an array of wind turbine rotors on a single tower that are individually optimized to improve the economics of the entire system.
Wind turbines have gained widespread use for electricity generation in recent years. The cumulative capacity of wind turbines installed worldwide has grown at a rate of approximately 32% per year over the past ten years. As of the end of 2001, the total installed capacity of wind turbines worldwide added up to over 20,000 MW. Future growth prospects for the industry are bright, although the economics of wind energy must continue to improve for the market to grow. There are signs that the potential for economic gains from current wind turbine technology is constrained.
As the market for wind turbines has grown in recent years, the size of the turbines has also grown. FIG. 1 shows the typical rotor diameter and rated power for state of the art wind turbines that have been installed in Europe over the past 15 years. Most wind turbine manufacturers have recently introduced turbine designs in the range of 1.5 to 2.5 MW with rotor diameters of 66 to 80 m. Even larger turbines are on the drawing boards of most wind turbine manufacturers. The trend toward larger turbines has been driven partly by technological and economic improvements, but the trend has largely been driven by market demand. Bigger wind turbines have proven to be more productive than smaller designs, largely due to the taller tower heights of the larger machines. Also, there is an economy of scale that favors large turbines because there are certain fixed costs associated with road construction, project planning, SCADA equipment, and operations and maintenance that do not increase with larger turbines. However, large turbines are also considerably more expensive than smaller machines and the economy of scale does not completely explain the trend to multi-megawatt size wind turbines. Project developers have demanded larger wind turbines at least partly due to perception issues. In Europe, where population density is relatively high compared to the United States, it is easier to obtain permits for fewer large turbines compared to a larger number of small turbines. Also, as wind turbines are installed offshore, there is a growing market for very large turbines to be used in that market.
As the size of wind turbines grows, there are several technical issues that adversely affect the economics of wind energy and that can potentially lead to constraints in turbine size. Basic design principles indicate that the weight of the turbine increases approximately with the cube of the rotor diameter. System cost is generally proportional to the turbine""s weight and so the cost of the turbine increases approximately with the rotor diameter cubed. The turbine weight and cost increase faster than the energy capture, which increases with the rotor diameter squared. For relatively small turbine sizes, there are other economies of scale that outweigh the increase in turbine weight and cost, however for turbines over approximately one megawatt in size the economies of scale are outweighed.
Another problem with large wind turbines is blade deflection. The wind turbine rotor is typically oriented upwind of the tower so that the blades bend downwind toward the tower. The turbine designer must take care so that the blade does not strike the tower, thereby causing catastrophic failure. The blade""s stiffness, defined by the material modulus of elasticity multiplied by the cross-sectional moment of inertia, or EI, increases as the blade becomes longer. However, the loads that cause deflection also increase for longer blades. If all of the blade""s dimensions are scaled proportionately to the blade length, then EI increases with the blade length to the fourth power whereas the bending moment increases with the blade length to the third power. This would lead to lower deflection for longer blades. However, practical considerations such as tooling, blade weight, and material cost constrain the design so that the blade""s chord and thickness are smaller relative to the blade""s length for large rotors. This causes a higher aspect ratio and lower solidity for large rotors. The lower solidity requires a higher tip speed for good aerodynamic performance and the higher tip speed can lead to increased centrifugal stiffening of the blade which reduces blade deflection. However, noise issues tend to constrain tip speed ratio so that centrifugal stiffening is less for very large rotors. Deflection thereofore becomes the design driver for very large rotors. Blade deflection can be mitigated by using large uptilt of the wind turbine nacelle. However, wind turbine designers are already using high (7 degrees) uptilt and negative coning to avoid tower strikes. Some blades are even being built curved to incorporate effective negative coning. All of this points toward blade deflection becoming a limiting design criteria for very large wind turbine rotors.
Another issue with very large rotors is that there is a large amount of composite material in each blade which can lead to material problems. Statistically, there is a higher probability of a defect existing in a large blade than in a small blade. If a defect is built into a blade, it can propogate to become a crack which will eventually lead to the blade""s failure. As the thickness of the blade""s laminate increases, it becomes more and more difficult to detect flaws in the material. Therefore, very large wind turbine blades may have a higher statistical probability of failure than a larger number of smaller blades.
Another issue for very large wind turbines is transportation and installation logistics. The long blade lengths being used on multi-megawatt wind turbines can exceed the capacity of public roads. Also, the tower heights necessary to support the large rotors can exceed the height capacity of cranes that are readily available.
Another problem experienced by the large wind turbines that are currently under development or being sold is that the rotors are so large that they experience a massive differential in wind speed from one side of the rotor to the other. Vertical wind shear exponents in the Midwest have been measured as high as 0.40 which can cause the wind speed across a 70 m rotor to vary by 62% from bottom to top if the turbine is mouted on a 65 m tower. The variation in wind loading is even more severe since loads are generally proportional to wind speed squared. In the example given, the bending load due to the wind at the top of the rotor would be 262% higher than the bending load due to the wind at the bottom of the rotor. Since each blade moves through this shear field, they are subjected to extreme fatigue loading conditions. From an energy standpoint, things are even worse. Energy in the wind is proportional to wind speed cubed which means that the energy content in the wind at the top of the rotor is 425% higher than the energy content at the bottom of the rotor.
Since all of the blades have the same rotational speed and pitch angle, it means that the entire rotor must be optimized for an average wind that is xe2x80x9cseenxe2x80x9d by the entire rotor. The rotor speed and blade pitch that work best for the wind speed at the rotor""s center may not work well at all for the portions of the rotor at the top and bottom. Therefore, at least part of the rotor will be operating in a sub-optimal condition whenever a wind shear is present. This problem is made worse as the turbine""s rotor diameter gets larger.
The issue of windspeed variation across the rotor also has negative implications for selecting an appropriate turbine for a given site. Wind turbine manufacturers generally offer their equipment with a range of rotor diameters for a given power rating or, conversely, with a range of power ratings for a given rotor diameter. For example, a 750 kW turbine may be sold with an option for a 46 m, 48 m, or 50 m rotor for high wind, moderate wind, and low wind sites respectively. Conversely, a company may offer a variety of power trains and generators with various power ratings for a turbine with a fixed rotor size. For example, a wind turbine with a 48 m rotor may be sold as a 600 kW, 700 kW, or 800 kW turbine for low speed, moderate speed, or high wind speed sites respectively. When the wind turbine""s rotor grows to be very large, it is more difficult to tailor the power rating and rotor diameter to fit the site. A turbine with a 70 m rotor diameter may experience an annual average wind speed of 6 m/s at the bottom tip of the rotor, 8 m/s at hub height, and 10 m/s at the upper tip of the rotor. Based on the annual average wind speed at the bottom tip of the rotor the turbine should be optimized for a low wind speed site, based on the annual average wind speed at hub height the turbine should be optimized for a moderate wind speed, and based on the annual average wind speed at the upper tip of the rotor the turbine should be optimized for a high wind speed site. Whichever rotor is selected, it will not be optimized for the entire rotor disk area.
As wind turbines grow very large there are several problems which need to be solved. First, the weight and cost of the turbine grow disproportionately for a very large rotor diameter. It would be desirable to provide a multi-megawatt wind turbine with a weight per unit of rotor swept area that is comparable to smaller turbine designs. Second, blade deflection becomes a problem and limits the rotor design for very large wind turbines. It would be desirable to provide a multi-megawatt wind turbine in which blade deflection is not a problem. Third, large wind turbine rotors have a greater statistical probability of material defects in the blades compared to smaller wind turbines. It would be desirable to provide a multi-megawatt wind turbine that does not require massive amounts of material in the blade roots leading to higher statistical probability of material defects. Fourth, transportation and construction logistics are problematic for very large wind turbines. It would be desirable to provide a multi-megawatt wind turbine that does not utilize massive blades and other components so that they can be easily transported and erected. Fifth, large wind turbines experience massive wind speed variations across their rotors so that at least a portion of the rotor is likely to be operating in sub-optimal conditions for the wind speed it is experiencing. It would be desirable to provide a multi-megawatt wind turbine in which the entire rotor area is optimized for the wind speed that it xe2x80x9csees.xe2x80x9d
The present invention solves the problems of the prior art wind turbines by utilizing a plurality of smaller rotors mounted on a single tower. The rotors are at various heights on the tower so that they each xe2x80x9cseexe2x80x9d different wind speeds. Accordingly, each rotor is optimized for its wind speed.
The individual rotors are more cost-effective than one massive rotor in that the combined weights and costs of the plurality of small rotors are less than the weight and cost of one massive rotor. Furthermore, because the rotors are mounted on a single tower, only one foundation, access road, and electrical connection are required, thereby providing cost savings over a plurality of smaller turbines on individual towers. Therefore, the turbine of the present invention provides the economy of scale that can be obtained with a very large turbine, but it also avoids the disadvantages associated with a massive rotor.
Another advantage of the present invention is that each of the plurality of rotors is optimized for its individual wind regime. Each rotor can have a unique power rating relative to its swept area. In this way, the blades, hubs, pitch assemblies, and main bearings are similar for all of the rotors and are interchangeable as spare parts. However, the drive train and generator for each rotor would be unique. Other parameters that could be optimized for each rotor include its solidity and tip speed, although if the rotors are to be interchangeable then the solidity must be consistent among rotors. Generally, the rotors toward the top of the tower have a higher power rating, a higher tip speed, and optionally a lower solidity. This allows each rotor to extract the maximum possible amount of energy out of the wind for the wind resource that it xe2x80x9csees.xe2x80x9d The energy extracted by a plurality of small rotors is greater than the energy extracted by a single massive rotor because each of the smaller rotors can be better tailored to its unique wind resource.
Each of the rotors is also controlled individually for the local wind speed that it experiences. Control parameters can include cut-in, cut-out, rotor speed, and blade pitch. By controlling each rotor individually it is possible to achieve a higher overall efficiency compared to controlling a single massive rotor based on the average wind speed that the rotor sees. Each rotor is controlled to be at the appropriate rotor speed and blade pitch to maintain peak efficiency. This allows the entire system to operate at peak efficiency for the entire range of wind speeds experienced from the lowest rotor to the highest rotor. In contradistinction, a single large rotor can only be controlled to operate at peak efficiency for one height and one wind speed while much of the rotor operates at lower efficiency.
The present invention has further advantages in terms of availability and maintenance. When a very large wind turbine faults offline, its entire production is lost. In contrast, one of the rotors of the present invention can fault offline with the resulting loss of only a small fraction of the total output. For example, if a 1.5 MW wind turbine experiences a blown fuse or some other relatively minor failure, the entire turbine is shutdown with the loss of 1.5 MW of power production. A comparable wind turbine system according to the present invention may include 15 rotors each with an output of 100, 200, 300, or 400 kW upon each rotor""s hub height in the vertical array. If a fuse or other minor part fails even in the uppermost rotor, then the lost output is only 400 kW and the system can continue to produce 3500 kW out of the total system rated power of 3900 kW.
From a maintenance point of view, the present invention""s larger number of small rotors allows the operators to keep a more complete selection of spare parts. For instance, if an operator is in charge of a 30 MW wind farm that consists of 20 turbines each rated at 1.5 MW they are not likely to keep any spare blades, generators, or gearboxes on hand. If one of those components experiences a failure, the operator must wait for the turbine manufacturer to supply another part and the turbine may be shut down for weeks. By contrast, if the operator was in charge of a 30 MW wind farm consisting of 20 turbines according to the present invention where each turbine has 15 rotors rated at 100 to 400 kW each, the operator would have a total of 300 sets of blades, generators, and gearboxes in operation. He could more easily justify having a spare set of components on hand because the cost of the set of spares would be lower in relation to the total cost of the wind farm.
Another maintenance advantage of the present invention is that it does not require a massive crane as prior art turbines do. Turbines that are being erected today in the 1.5 to 2 MW size range require cranes with capacities of over 500 tons that are very expensive to mobilize. By comparison, each of the smaller rotors on the wind turbine of the present invention can be lifted using a much smaller crane that is locally available and can be mobilized for a small fee. Maintenance can be further facilitated if each array of wind turbine rotors includes a boom-car crane. The boom-car crane could be located on top of the tower and could be used to remove and replace any of the rotors without the assistance of a separate crane.